1. Field of the Invention
The present invention relates generally to cooling of turbine rotor blades and stator vanes in gas turbine engines and, more specifically, to serpentine cooling circuits therein.
2. Discussion of the Background Art
A gas turbine engine includes a compressor that compresses air which is channeled to a combustor wherein it is mixed with fuel and ignited for generating combustion gases. The combustion gases flow downstream through one or more stages of turbines which extract energy therefrom for powering the compressor and producing additional output power for driving a fan for powering an aircraft in flight for example. A turbine stage includes a row of turbine rotor blades secured to the outer perimeter of a rotor disk, with a stationary turbine nozzle having a plurality of stator vanes disposed upstream therefrom. The combustion gases flow between the stator vanes and between the turbine blades for extracting energy to rotate the rotor disk. Since the combustion gases are hot, the turbine vanes and blades are typically cooled with a portion of compressor air bled from the compressor for this purpose. Diverting any portion of the compressor air from use in the combustor necessarily decreases the overall efficiency of the engine. Accordingly, it is desired to cool the vanes and blades with as little compressor bleed air as possible.
Typical turbine vanes and blades include an airfoil over which the combustion gases flow. The airfoil typically includes one or more serpentine cooling passages therein through which the compressor bleed air is channeled for cooling the airfoil. The airfoil may include various turbulators therein for enhancing cooling effectiveness and, the cooling air is discharged from the passages through various film cooling holes disposed around the outer surface of the airfoil.
The temperature profile of the combustion gases channeled over the airfoil is typically center peaked at about 50% to about 80% of the radial height or span of the airfoil. Secondary flow fields between adjacent airfoils may sometimes cause the temperature profile of the combustion gases to shift radially outwardly on the pressure side of the airfoil. Accordingly, the airfoil typically experiences relatively high heat input loading on its pressure side above the airfoil mid-span. Since the serpentine cooling circuits introduce air into the airfoil from its root, the cooling air must be provided with a suitable flow rate to ensure that the outer portions of the airfoil experiencing the greatest heat input are adequately cooled for obtaining a useful life during operation. The inner portions of the airfoil may therefore be over-cooled which is an inefficient use of the valuable compressor bleed air. To overcome this drawback a "Multi-tier turbine airfoil", disclosed in U.S. Pat. No. 5,591,007, was devised and is incorporated herein by reference. This patent discloses a turbine airfoil having a plurality of internal ribs defining at least two independent serpentine cooling circuits arranged in part in different longitudinal tiers, with an outer tier circuit being disposed in part longitudinally above an inner tier circuit for differentially longitudinally cooling the airfoil. More advanced turbine airfoil designs have been developed that could use a better cooling air distribution.
Typical mid-circuit cooling air, after picking up the heat in the serpentine passage, exits through film cooling holes. One or more rows of film cooling holes are placed on the pressure side and also on the suction side. New highly aerodynamically efficient airfoils in low through flow turbine designs are subject to an external gas path flow along the pressure side that has low velocity. This can result in a very high blowing ratio (mass flux ratio of film cooling air to gas flow) through the film cooling holes and very poor film cooling effectiveness (film blow-off) on the pressure side of the airfoil. Geometrical limitations of at least some of the cavities which supply the film cooling air prevent or make difficult the use of film holes on both pressure and suction sides that have relatively shallow angles from the surfaces of the sides. The use of larger angles would result in significant aerodynamic mixing losses and poor film cooling effectiveness because much of the film cooling air would flow out of the boundary layer. Therefore, it is desirable to have a circuit design which can avoid the use of film cooling in such areas of the airfoil and provide effective and efficient film and convective cooling of the entire airfoil.